Various systems have been developed to track the movement of and location of railway trains on track systems.
In its simplest form, train position can be ascertained at a central control facility by using information provided by the crew, i.e., the train crew periodically radios the train position to the central control facility; this technique diverts the attention of the crew while reporting the train position, often requires several “retries” where the radio link is intermittent, and the position information rapidly ages.
Early efforts have involved trackside equipment to provide an indication of the location of a train in a trackway system. Wayside devices can include, for example, various types of electrical circuit completion switches or systems by which an electrical circuit is completed in response to the passage of a train. Since circuit completion switches or systems are typically separated by several miles, this technique provides a relatively coarse, discrete resolution that is generally updated or necessarily supplemented by voice reports by the crew over the radio link.
In addition, information from one or more wheel tachometers or odometers can be used in combination with timing information to provide distance traveled from a known start or waypoint position. Since tachometer output can be quite “noisy” from a signal processing standpoint and accuracy is a function of the presence or absence of wheel slip, the accuracy of the wheel-based distanced-traveled information can vary and is often sub-optimal.
Other and more sophisticated trackside arrangements include “beacons” that transmit radio frequency signals to a train-mounted receiver that can triangulate among several beacons to determine location.
While trackside beacon systems have historically functioned in accordance with their intended purpose, trackside systems can be expensive to install and maintain. Trackside systems tend not to be used on a continent-wide or nation-wide basis, leaving areas of the track system without position-locating functionality (viz., “dark” territory).
More recently, global navigation satellite systems such as the Global Positioning System (GPS) and the nationwide Differential GPS (NDGPS), have been used to provide location information for various types of moving vehicles, including trains, cargo trucks, and passenger vehicles. GPS and similar systems use timed signals from a plurality of orbital satellites to provide position information, and, additionally, provide accurate time information. The time information can include a highly accurate 1 PPS (1-pulse-per-second) output that can be used, for example, to synchronize (or re-synchronize) equipment used in conjunction with the GPS receiver. The GPS/DGPS receivers require a certain amount of time to acquire the available satellite signals to calculate a positional fix. While the GPS system can be used to provide position information, GPS receivers do not function in tunnels, often do not function well where tracks are laid in steep valleys, and can fail to operate or operate intermittently in areas with substantial electromagnetic interference (EMI) and radio frequency interference (RFI). When a GPS system is operated on a fast-moving vehicle, the location information becomes quickly outdated. In addition, the accuracy of the GPS system for non-military applications is such that track occupancy (which track a train is on among two or more closely spaced tracks) cannot be determined consistently and reliably.
Current philosophy in train systems is directed toward higher speed trains and optimum track utilization. Such train systems require ever more resolution in train location and near real-time or real time position, distance from a known reference point, speed, and direction information. In addition to locating a train traveling along a particular trackway to a resolution of one or two meters, any train location system should be able to locate a train along one of several closely spaced, parallel tracks. Since track-to-track spacing can be as little as three meters, any train location system must be able to account for train location on any one of a plurality of adjacent trackways or determine track occupancy at a turnout or other branch point.